1. Field of the Invention
The present invention relates to a device for machining a workpiece by removing chips, forming part of a machine tool and comprising a positioning and longitudinal-displacement means. This may involve machining a workpiece, for example made of metal, be it cylindrical (by turning, drilling, shaving, cylindrical grinding) or not (by milling, drilling, grinding) with a view to obtaining a workpiece of predetermined size and shape.
2. Description of the Prior Art
The machine tools or machines for machining known to date consist of a support base on the ground, supporting or incorporating a bed for supporting the guide elements. These guide elements are made in the form of slideways of parallelepipedal--generally rectangular--cross section, which are not very stiff and therefore have to be supported along their entire length by a bed which thus gives the machine its precision and rigidity. This bed thus generally covers the entire surface of the machine and constitutes an obstacle to the efficient removal of chips.
To overcome this drawback, certain turning machines have been designed with a bed which is inclined to the operator or which is vertical. In this configuration, the latter constitutes an impediment to loading or unloading the machine. In particular, when installing automated production lines:
it makes it impossible to obtain a natural flow of workpieces when the exit point differs from the entry point, which means that factory layouts are not as optimized as they might be; PA1 it means that the feed robot has to be installed on the same side of the machine as the operator, which results in difficulties with access for settings or maintenance. PA1 either designing a bed which has chip retainers and requires the use of telescopic protectors which are incompatible with good operational reliability in a mass-production environment; PA1 or to using a key system, or any other currently known system for preventing rotation which, depending on its position: PA1 either a guide column alone, supporting a spindle with built-in motor, for example for turning, milling, grinding, which spindle may have a number of degrees of freedom with respect to the movement of the column; PA1 or a guide column alone supporting a tool-holder turret which may have a number of degrees of freedom with respect to the movement of the column; PA1 or a built-in fixed spindle or a guide column supporting a spindle with a built-in motor, together with one or more guide columns supporting a tool-holder turret which may have a number of degrees of freedom.
As they lie in the flow of chips and cutting fluids, these beds and their guide systems require protection. This protection, which has to be telescopic to allow for the movements of the machine, and therefore very expensive, is not readily compatible with high speeds and accelerations. Used in a mass-production environment, for example for machining parts for the automotive industry, these protection elements quickly deform and thus offer passages through which the chips can pass, which has an adverse effect on the reliability and life of the machine. Once they have been in use for a certain amount of time, they act more as an obstacle to removing and cleaning away the chips than as a true barrier against the mechanical aggression of these chips.
Certain turning machines have been designed with a bed facing back towards the rear of the headstock, while at the same time maintaining guidance on slideways. Protection demarcating the chip zone from the mechanical-precision zone is still, however, very tricky to achieve and cleaning out chips which have managed to get in is almost impossible.
Furthermore, because of their presence, these beds fix the configuration of the axes, of the spindles and of the movements of the tools or workpieces with respect to the floor, and their relative positions.
In multi-spindle milling or punching-drilling machines, it is very important, for maintaining precision under the force exerted during cutting, that the resultant of the forces exerted on the guides by the cutting force be centred with respect to the guides so that they do not cause any deflection of the position of the axis of the spindle. This is very tricky to achieve with straightedge guides, or leads to U-shaped straightedge support structures which are therefore open and not very rigid.
A guide profile must have mechanical properties that are adequate for obtaining correct operation and good precision of the machines, namely:
1--It must have maximum bending stiffness to withstand the cutting forces and form, with the mass of the turret system for changing tools and controlling additional axes (for example the X, Y, Z axes) with the tool fixed at the end of the spindle and the spindle of a machining centre or with the mass of any other machining or workpiece holder system fixed at its end, an assembly whose natural frequency of vibration is high enough that it does not disrupt the cutting process. PA0 2--Its torsional stiffness must be high enough that, while withstanding the overturning moment that is due to the cutting force, does not affect the precision of the machine. PA0 3--Its mass must be low so that speeds and accelerations compatible with the current design of so-called "agile" machines, namely a speed of 1 m/s and an acceleration of 10 m.s.sup.-2 can be obtained economically. PA0 4--Its shape must be simple, cylindrical, and its profile must be as constant as possible so that it can be machined by surface grinding and so that hydrostatic guide bearings can be developed (eliminating wear), and so that effective protection against the ingress of chips into the bearings can be achieved simply by scraping over a continuous profile. PA0 5--Its shape must be able, without the addition of an additional guidance system, to angularly position the system fixed to the end of this column.
Points 1, 2, 3 and 4 are advantageously optimized by the use of a cylindrical column of circular profile, this having maximum moment of inertia for minimum mass. This solution has already been used to produce guides fixed to a bed. By contrast, as it requires an additional angular-positioning system, it does not satisfy point 5, a condition which is essential for simplifying chip protection.
Specifically, additional prismatic guidance amounts to:
at the front: constitutes a difficulty with scraping and sealing which is not compliant with point 4, PA2 at the rear: constitutes an asymmetry in the way that torque is taken up, thus creating eccentricity and bending in the column, which is detrimental to the high precision desired.
Document U.S. Pat. No. 4,604,008 A discloses a machining device comprising, in a conventional chassis, a longitudinal-displacement means with rotation being prevented, using a four-lobed cross section, the assembly itself being capable of being rotated. Document U.S. Pat. No. 3,263,530 A discloses a head of a precision boring device of four-lobed cross section. Document EP 0 172 299 A discloses a compact machine tool. None of these is able to dispense with a bed.